Mapping ultrasound transducers

ABSTRACT

Ultrasound transducers may be mapped by varying a focus-affecting parameter and adjusting the parameter so as to improve focus quality. In some embodiments, mapping involves successively varying the phase of one transducer element, or group of elements, with respect to a constant phase of the other transducer elements, and determining the phase at which a tissue displacement in the ultrasound focus is maximized.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to and the benefit of U.S.Provisional Patent Application No. 61/251,450, filed on Oct. 14, 2009,the entire disclosure of which is hereby incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates, generally, to systems and methods formapping ultrasound transducers. In particular, various embodiments aredirected to improving the quality of the ultrasound focus usingexperimental feedback.

BACKGROUND

Focused ultrasound (i.e., acoustic waves having a frequency greater thanabout 20 kilohertz) can be used to image or therapeutically treatinternal body tissues within a patient. For example, ultrasonic wavesmay be used to ablate tumors, eliminating the need for the patient toundergo invasive surgery. For this purpose, a piezo-ceramic transduceris placed externally to the patient, but in close proximity to thetissue to be ablated (“the target”). The transducer converts anelectronic drive signal into mechanical vibrations, resulting in theemission of acoustic waves. The transducer may be shaped so that thewaves converge in a focal zone. Alternatively or additionally, thetransducer may be formed of a plurality of individually driventransducer elements whose phases can each be controlled independentlyfrom one another. Such a “phased-array” transducer facilitates steeringthe focal zone to different locations by adjusting the relative phasesbetween the transducers. Magnetic resonance imaging (MRI) may be used tovisualize the patient and target, and thereby to guide the ultrasoundbeam.

The effectiveness of ultrasound therapy depends on the accuracy of thefocus location, the sharpness and shape of the focal zone, and theavoidance of “hot spots” (i.e., regions of high ultrasound intensity)outside the target. Transducer elements that are not properly configuredor controlled can lead to improper focus location and reduced focusquality, resulting in less effective therapy, and possibly damage tohealthy tissue surrounding the target. It is therefore desirable tocorrect any mechanical misconfigurations. Improper transducerconfiguration may result from manufacturing errors, inadvertent shiftingof transducer elements from their expected locations during use orrepair, deformation of the transducer due to thermal expansion, or acombination of these and other effects. Even slight locationaldeviations can have significant effects on the quality of the transduceroutput. For example, as illustrated in FIG. 1, if the height of a curvedtransducer surface having a width of 120 mm and a nominal radius ofcurvature of 160 mm changes by only 1 mm, the ultrasound focus shifts byabout 13 mm. In a phased array, deviations of the transducer locationsfrom the intended locations can be compensated for by adjusting thephases with which the elements are driven. This procedure is hereinafterreferred to as “mapping” the transducer.

One approach to mapping a phased-array transducer surface involvesdriving each transducer element individually to produce an acoustic wavepulse in water; measuring the arrival of the acoustic wave pulse inthree locations using a hydrophone; determining for each location thetime of flight, and thus the distance, from the transducer element; andcalculating the coordinates of the element location by triangulationfrom the three measurements. Based on the intended and the measuredactual locations of the transducer elements, the necessary phaseadjustments can be calculated. This method is described in U.S. Pat. No.7,535,794 to Prus et al., which is hereby incorporated herein byreference in its entirety. In addition to a hydrophone, implementationof the method requires other auxiliary equipment, such as an amplifierand data-acquisition module. Further, to avoid damaging the hydrophone,the mapping is typically performed at transducer power levelssignificantly below those used during normal operation, which canundermine the validity of the adjustments under therapeutic conditions.Alternative transducer mapping methods that do not have these drawbacksare therefore desirable.

SUMMARY

The present invention generally provides methods for mapping aphased-array transducer by generating an ultrasound focus and improvingthe focus quality based on experimental feedback. In variousembodiments, one or more focus-affecting parameters (such as the phaseand/or amplitude of one or more transducer elements) are varied, and theresulting variation of the focus quality is measured (e.g., in terms ofan integral or peak intensity, focus size, or intensity profile). Thefocus-affecting parameter(s) are then set to values for which the focusquality is optimized. For example, the relative phases of the transducerelements may be fine-tuned one at a time to maximize the intensity atthe focus. In this manner, unwanted phase shifts resulting fromelectronic delays or other sources may be corrected, and any deviationsfrom the intended locations of the transducer elements may becompensated for without the need to explicitly determine the actualtransducer locations.

During the mapping procedure, the ultrasound focus may be generated in aphantom. To determine the focus quality, the focus may be visualized,for example, by magnetic-resonance acoustic radiation force imaging(MR-ARFI)—an MRI technique measuring minute material displacements thatare caused by and indicative of the acoustic field. The displacementincreases with the acoustic field intensity. Thus, by adjusting thetransducer element phases (and/or amplitudes or other focus-affectingparameters) so as to increase the material displacement in the phantom,the intensity at the focus and, consequently, the focus quality may beimproved. Advantageously, MR-ARFI facilitates mapping the transducer atnormal operational power levels, which increases the relevance andapplicability of any mapping-based adjustments to the subsequenttherapeutic operation. Further, the ultrasound focus may be imagedduring the mapping procedure with the same MRI or other imagingapparatus (e.g., X-ray-based computer-aided tomography or othertomographic modality) that is used to guide the focus during therapeuticoperation, and, consequently, additional (auxiliary) mapping equipmentis not needed.

In one aspect, the invention provides a method for improving and/oroptimizing the focus of an ultrasound transducer having a plurality oftransducer elements. The method includes driving the plurality oftransducer elements so as to generate an ultrasound focus, varying afocus-affecting parameter associated with one or more of the transducerelements, measuring a resulting variation on the quality of the focus,and selecting parameter value(s) that result in the best focus quality.The focus-varying parameter(s) may be or include the phase and/oramplitude of one of the transducer elements (or a group of jointlydriven elements), or a phase/amplitude gradient or other parameterdetermining relative phase/amplitude settings of multiple transducerelements that form the whole transducer or a region thereof, or acombination of such parameters. In embodiments in which the locationand/or orientation of the transducer or transducer elements aresusceptible to direct user control, the focus-varying parameter(s) may,alternatively or additionally, include such location(s) and/ororientation(s). Other focus-varying parameters include, e.g., the drivefrequency of the transducer. The quality measurement may involve, forinstance, scanning a profile of the focus (e.g., measuring the intensityin the focus region along a line through the focus), measuring the peak(i.e., maximum) intensity of the focus, and/or measuring the integralintensity or size of the focus (i.e., integrating the intensity or areaover the cross-section of the focus, where the cross-section is defined,e.g., by the regions in which the intensity is more than a set fraction,e.g., half or 1/e, of the peak intensity).

In certain embodiments, varying the focus-varying parameter(s) andmeasuring the resultant focus quality includes driving a selected one ofthe transducer elements at a variable phase while driving the othertransducer elements at a constant phase (thereby varying the focusquality); determining the phase difference, if any, between the constantand variable phases where the focus quality is optimized; and, if thephase difference is non-zero, adjusting the relative phase of theselected transducer element based thereon. These steps may be repeatedfor the remaining transducer elements.

In some embodiments, the quality measurement comprises measuring thedisplacement associated with the focus using, e.g., ARFI. The ARFImeasurement may involve applying a sequence of MR field gradients (suchas, e.g., repeated bipolar gradients). The ultrasound focus maygenerated by an ultrasound pulse synchronized with the sequence of MRfield gradients. The method may further involve providing a phantom(which may include a material having low tensile strength and/or a smallelastic modulus), and generating the ultrasound focus and measuring thefocus quality in the phantom.

In another aspect, the invention is directed to a method for mapping anultrasound transducer comprising a plurality of transducer elements. Themethod includes driving the transducer elements so as to generate anultrasound focus, and measuring a displacement associated with the focusby ARFI. Further, it involves varying the displacement by driving one ofthe transducer elements at a variable phase while driving the othertransducer elements at a constant phase, and determining the phasedifference, if any, between the constant and variable phases at whichthe displacement is maximized. If the phase difference is non-zero, therelative phase of the selected transducer element is adjusted based onthe phase difference. The phase difference may also be used to determinethe location of the selected transducer element. The application of avariable phase and determination of the phase difference between thevariable and constant phases may be repeated for the remainingtransducer elements. The ultrasound focus may be generated, and thedisplacement be measured, in a phantom, which may have a low tensilestrength and/or a small elastic modulus. Acoustic-radiation forceimaging to measure the displacement may involve applying a sequence ofMR field gradients, e.g., a sequence including repeated bipolargradients. The transducer elements may be driven so as to generate anultrasound pulse synchronized with the sequence of MR field gradients.

The above-described mapping method(s) may be varied by groupingtransducer elements and mapping a group, rather than an individualelement, at a time. Accordingly, in yet another aspect of the invention,a selected group of transducer elements is driven at a variable phasewhile other groups of transducer elements are driven at a constantphase, and the phase difference between the constant and variable phasesthat maximizes the displacement associated with the focus may bedetermined and form the basis for adjusting the relative phase of theselected group of transducer elements.

In a further aspect, the invention is directed to a method forcontrolling an ultrasound transducer having a plurality of transducerelements. The method includes mapping the ultrasound transducer asdescribed above by driving the transducers to generate an ultrasoundfocus; varying a focus-affecting parameter associated with at least oneof the elements and measuring a resulting variation on a quality of thefocus; and selecting a value of the parameter associated with a bestfocus quality. In particular, in some embodiments, mapping may includemeasuring a displacement associated with the focus (e.g., using ARFI),and determining, for each of the transducer elements, the relative phaseassociated with that element by driving the transducer element at avariable phase while driving the other transducer elements at a constantphase and determining the phase difference between the constant andvariable phases that maximizes the displacement. The method furtherincludes controlling the transducer based on the mapping step (e.g., byadjusting the relative phase of each transducer for which the phasedifference is non-zero). Controlling the transducer may include drivingthe elements to produce outputs converging at a focus corresponding to atarget treatment region. Mapping may be carried out using a phantom, andmay be repeated in between therapeutic applications of ultrasound.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be more readily understood from the followingdetailed description of the invention in conjunction with the drawings,wherein:

FIG. 1 is a schematic drawing illustrating the effect of a change in thetransducer surface on the focus location;

FIG. 2 is a block-diagram illustrating, on a high level, a system forperforming transducer mapping in accordance with some embodiments of theinvention;

FIG. 3 a schematic diagram illustrating, in more detail, theconfiguration of a transducer mapping system in accordance with someembodiments;

FIGS. 4A-4C illustrate various MR-ARFI sequences in accordance with someembodiments;

FIG. 5 is an image of material displacements in an ultrasound focusregion in accordance with some embodiments;

FIG. 6 is a graph illustrating material displacement in the focus centeras a function of the phase of an individual transducer element, as itmay be used in mapping methods in accordance with various embodiments;and

FIG. 7 is a flow chart illustrating an approach to mapping an ultrasoundtransducer array in accordance with various embodiments of theinvention.

DETAILED DESCRIPTION

FIG. 2 illustrates in block-diagram form an exemplary system forperforming transducer mapping in accordance with various embodimentshereof, and the interplay between the different system components. Thesystem includes, first, the focused ultrasound transducer hardware 202,which itself includes the transducer array, a frequency generator forproviding an electronic drive signal, and drivers for the transducerelements. (As used herein, the term “transducer” refers to the entirearray, as distinguished from the individual transducer elements.) Eachdriver contains electronic circuitry for setting the phase of therespective transducer element(s), and optionally also for adjusting theamplitude(s) of vibrations. In some embodiments, each transducerelements is independently controllable by a separate associated driver.In alternative embodiments, driver elements are organized (e.g., viahardwiring or configurable switches) into multiple groups of elementsdriven collectively by the same driver. The drivers are controlled by acontrol station 204, which may include a computer system speciallydesigned for use with the transducer hardware, or a general-purposecomputer (or cluster of computers) having suitable (and conventional)driver software installed.

The mapping system 200 further includes MRI (or other tomographic orimaging) hardware 206, i.e., an MRI apparatus and related drivercomponents, which is likewise controlled by the control station 204.Again, control functionality for the MRI hardware 206 may be implementedin a special-purpose computer system, or in conventional driver softwareinstalled on a general-purpose computer system. The transducer hardware202 and MRI hardware 206 may be controlled by the same computer withincontrol station 204, or by separate computers that are in communicationwith one another. Further, computational functionality for processingand analyzing the images acquired with the MRI hardware 206 may beintegrated with the MRI apparatus, or implemented (e.g., as a separatesoftware module) in control station 204.

During the mapping procedure, the control station 204 sends controlsignals to the ultrasound transducer hardware 202 to vary one or moreparameters affecting the focus properties. For example, the controlstation 204 may cause a phase modulation of a particular transducerelement or group of elements. Further, the control station 204 providesscan parameters, or other signals triggering and/or controlling imageacquisition, to the MRI or other imaging hardware 206. The relativetiming of the ultrasound generation and/or modulation with respect toimage acquisition may be specified in an imaging sequence, which may beprogrammed into the control station 204. The control station 204 maythen send trigger signals to the ultrasound transducer hardware 202 andthe MRI hardware 206, ensuring correct timing between the signals inaccordance with the imaging sequence. Alternatively, the control station204 may communicate a time-delay parameter, which specifies the timedelay between the RF pulses and ultrasound pulses, to the MRI hardware206, which sends a corresponding trigger pulse directly to theultrasound transducer hardware 204.

The acquired images are processed to determine one or more parameter(s)indicative of the focus quality, which are used in the control station204 for subsequent mapping steps. For example, in MR-ARFI-based systems,a material displacement indicative of the intensity in the focus iscomputationally extracted from the images, and the transducer isiteratively adjusted so as to increase the material displacement.Similarly, if thermal MRI is employed, the mapping process involvesmaximizing the temperature, and thus intensity, in the focus. Ingeneral, any imaging technique that provides images suitable fordetermining focus quality may be used. Depending on the contemplatedultrasound application, the “quality” of the focus may be expressed bydifferent parameters. For therapeutic applications involving targetedtissue destruction, for example, the focus quality may be measured interms of a peak intensity or total power delivered (corresponding to anintegrated intensity over the focus cross section). If the target areais small, the focus size may be relevant (a smaller focus area generallycorresponding to higher focus quality). In some applications (e.g.,tissue heating for palliative purposes), homogeneity of the intensityfocus area may be important, and focus quality may, accordingly, bemeasured, at least in part, by the “smoothness” of an intensity profilethrough the focus.

FIG. 3 schematically illustrates an experimental setup for mapping anultrasound transducer using tissue displacement as an indicator of focusquality. During the mapping procedure, the transducer 300 is driven soas to focus an ultrasound wave pulse into a phantom 302, which comprisesor consists of a material that responds to acoustic pressure in adetectable manner. The ultrasound wave exerts acoustic radiationpressure onto the phantom material along its path. At the focus 304,where waves from the individual transducer elements converge, thispressure is highest, resulting in a temporary local displacement of thematerial in the longitudinal direction and/or in shear waves thatpropagate radially away from the focus. By using a soft phantom thatresponds to acoustic pressure with large enough shear strain (e.g., witha shear strain of at least 10⁻²), a detectable displacement field thatdirectly reflects the acoustic field may be obtained. Suitable phantommaterials have high tensile strength (e.g., higher than 100 kPa) andsmall elastic moduli (e.g., Young's modulus less than 1 MPa), and mayinclude or consist of jelly-like materials such as, e.g., Silicone GelRTV6166, provided by General Electric Co., Waterford, N.Y.

The material displacement may be visualized in an imaging plane 306using an imaging techniques such as, e.g., magnetic-resonance-basedacoustic radiation force imaging (MR-ARFI). In MR-based imaging methods,the object to be imaged (here, the phantom) is placed in a relativelyuniform static magnetic field having a field strength of, typically,between about 1.5 and about 3.0 Tesla. Such a field may be generated,for example, by a large cylindrical electromagnet coil 308. The staticmagnetic field causes hydrogen nuclei spins to align and precess aboutthe general direction of the magnetic field. Radio frequency (RF) pulsesand magnetic gradients are then superimposed on the static magneticfield to cause some of the aligned spins to alternate between atemporary high-energy non-aligned state and the aligned state, therebyinducing an RF response signal, called the MR echo or MR responsesignal, in the RF antenna 310.

In MR-ARFI, transient-motion or displacement-sensitizing magnetic fieldgradients are applied to the phantom by gradient coils, which are partof standard MRI systems and are typically located near the cylindricalelectromagnet coil 308. When the ultrasound pulse is applied in thepresence of such gradients, the resulting displacement is directlyencoded into the phase of the MR response signal. For example, thegradient coils and transducer may be configured such that the ultrasoundpulse pushes phantom material near the focus towards regions of themagnetic field with higher field strengths. In response to the resultingchange in the magnetic field, the phase of the MR response signalchanges proportionally, thereby encoding in the signal the displacementcaused by the ultrasound radiation pressure.

To achieve high image contrast, the ultrasound pulse, encodinggradients, and RF pulse are precisely timed with respect to each otheraccording to a suitable displacement-encoding sequence. FIGS. 4A-4Cillustrate five exemplary MR-ARFI sequences that may be used inembodiments of the invention. These sequence diagrams illustrate theorder in which the displacement-encoding magnetic field gradients (thinsolid lines), ultrasound pulses (dotted lines), and RF pulses (thicksolid lines) appear in time. Three different field gradient sets areshown: two single lobes (a), repeated bipolars (b), and invertedbipolars (c). For gradient set (a), ultrasound may be applied duringeither the first or the second lobe. Similarly, for gradient set (c),ultrasound may be applied during the first or the second halves of thebipolars. In general, MR-ARFI sequences utilize magnetic field gradientsthat are synchronized with the ultrasound pulses. In preferredembodiments, a sequence like the repeated bipolar sequence (b) shown inFIGS. 4A-4C may be used.

An example of an MR-ARFI image of an ultrasound focus region is shown inFIG. 5. As shown, the material displacement with respect to anequilibrium position varies between about −1 μm and 5 μm, as indicatedin the color-coding of the image. In general, the stronger the acousticfield intensity, the greater will be the maximum displacement at thecenter of the focus. The acoustic field intensity, in turn, is maximizedwhen the elements of the ultrasound transducer emit acoustic waves thatare all in phase at the focus position. If a transducer element is outof phase with respect to the others, the focus intensity in the centerdecreases. This relationship can be exploited to optimize the focus, andthus to map and adjust the transducer elements. Assuming, for example,that all but one of the transducer elements are properly configured, thecorrect phase of the last element can be determined by tuning the phaseover a full cycle (e.g., between −π and +π), measuring for each phasethe displacement in the focus center, and then setting the phase to thevalue corresponding to the maximum displacement. FIG. 6 depicts theresults of such an adjustment procedure. In the illustrated example, thematerial displacement over the full phase cycle of one element variesbetween about 4.85 μm and about 5.4 μm. The maximum displacement occursat about 0.12 rad. Consequently, the focus intensity and quality can beimproved by introducing a phase shift of 0.12 rad for the testedtransducer element.

In certain embodiments, mapping of the full transducer array isaccomplished by varying and adjusting the phase (and/or amplitude) ofeach element, one at a time, while driving the remaining elements atconstant phase. Typically, after each element has been mappedindependently, the focus quality has significantly improved. Since thenecessary phase adjustments of the transducer elements are allinterrelated, however, the focus may not yet be optimal after oneiteration. Therefore, in some embodiments, the procedure may be repeatediteratively. With each iteration, the phase adjustments made to maximizethe displacement in the focus will, generally, decrease. Thus, atermination condition may be defined by setting a threshold value forphase adjustments, below which further adjustments are deemed immaterialor not clinically necessary. The number of iterations required to reachthe termination condition may depend on the order in which thetransducer elements are mapped. A mathematical algorithm, for example a“greedy algorithm” as known to persons of skill in the art, may be usedto select a mapping order that results in fast convergence of the phasesettings. In certain alternative embodiments, the transducer elementsmay be grouped, and groups of elements may be mapped simultaneously.

FIG. 7 illustrates a representative method for mapping an ultrasoundtransducer array in accordance with various embodiments of theinvention. The method involves, in a first step 700, providing aphantom, and installing the transducer array and phantom in an MRIapparatus. Further, an imaging sequence defining the duration andrelative times of ultrasound pulses, RF pulses, and encoding gradientsis chosen (step 701). The phases of the transducer elements are then setto a constant value (step 702), and the focus location is determined byidentifying the position of the phantom's maximal displacement in theMR-ARFI image (step 703). An iterative phase-adjustment process issubsequently started by selecting one element (or one group of elements)for mapping (step 704). The selected element is driven at a variablephase (step 708), and for each phase setting, the transducer elementsare collectively driven to generate an ultrasound focus in the phantom(710). Substantially simultaneously, or after a time intervaloverlapping with the generation of the ultrasound focus, thedisplacement of phantom material in the focus region is measured byMR-ARFI in accordance with the imaging sequence (step 712). Typically,this step is repeated until the variable phase of the selectedtransducer element has covered a range of phases spanning an interval of2π. In step 714, the phase difference between the constant phase and thephase of the selected transducer element that maximizes the displacementis determined (step 712). If the phase difference is non-zero, the phaseof the transducer element is adjusted by this difference (step 718).Steps 704 through 718 are then repeated until each transducer elementhas been mapped. If the phase adjustments made in step 718 fall below apre-determined threshold value, or an alternative termination conditionis satisfied, the mapping is complete. Otherwise, the mapping procedurefor the array may be repeated, starting with the adjusted phase settingsof the transducer elements.

The method described above may be varied in several ways. For example,the phase differences may first be determined for all the elements,without adjustments being made, and following this mapping procedure,all the phase adjustments may be made at once. In this case, aniterative phase adjustment is not needed because the reference phase,i.e., the phase of the transducer as a whole, disregarding the elementunder mapping, is nearly the same for all elements. Further, in someembodiments, the phases of the transducer elements may be adjustedsimultaneously, rather than in succession, by varying a drive parameteraffecting some or all of the elements. For example, the relative phasesbetween transducer elements may be expressed in terms of a functionaldependence of the phase on the position of a transducer element alongone or two axes of the transducer array, and such functional dependence,in turn, may be characterized by one or few mathematical parameters(e.g., a linear phase gradient and/or coefficients of higher-ordercomponents of the phase modulation in one- or two-dimensional space).Rather than modulating the phase of an individual element, then, thecoefficients in the functional dependence may be varied, and thecorresponding effect on the focus quality observed.

Alternatively or in addition to phase variations, amplitude variationsof individual elements or groups of elements (including variations ofthe functional dependence of the amplitude on a position along thearray) may also be employed to improve the focus quality. Further, insome embodiments, the transducer elements may be movable with respect tothe one another within certain ranges (e.g., as a consequence of beingmounted on electronically controllable microtranslator stages, pivots,etc.). The experimental adjustment procedure described above may then beused to fine-tune the positions and/or orientations of the transducerelements. The overall shape, position, and orientation of the transducermay likewise be controllable, e.g., via clasps, movable bearings, etc.

Although the present invention has been described with reference tospecific details, it is not intended that such details should beregarded as limitations upon the scope of the invention, except as andto the extent that they are included in the accompanying claims.

1. A method for improving the focus of an ultrasound transducercomprising a plurality of transducer elements, the method comprising:(a) driving the plurality of transducer elements so as to generate anultrasound focus; (b) varying a focus-affecting parameter associatedwith at least one of the elements and measuring a resulting variation ona quality of the focus; and (c) selecting a value of the parameterassociated with a best focus quality.
 2. The method of claim 1 whereinthe quality measurement comprises scanning a profile of the focus. 3.The method of claim 1 wherein the quality measurement comprisesmeasuring a size of a cross-section of the focus.
 4. The method of claim1 wherein the quality measurement comprises measuring an integralintensity of the focus.
 5. The method of claim 1 wherein the qualitymeasurement comprises measuring a peak intensity of the focus.
 6. Themethod of claim 1 wherein the quality measurement comprises measuring adisplacement associated with the focus.
 7. The method of claim 6 whereinthe displacement is measured using acoustic radiation force imaging. 8.The method of claim 7 wherein the acoustic radiation force imagingcomprises applying a sequence of MR field gradients.
 9. The method ofclaim 8 wherein the sequence of MR field gradients comprises repeatedbipolar gradients.
 10. The method of claim 8 wherein step (a) comprisesgenerating an ultrasound pulse synchronized with the sequence of MRfield gradients.
 11. The method of claim 1 wherein the focus-affectingparameter comprises at least one of a phase or an amplitude of the atleast one element.
 12. The method of claim 1 wherein the focus-affectingparameter comprises at least one of a location or an orientation of theat least one element.
 13. The method of claim 1 wherein the varying andmeasuring step comprises: (i) driving a selected one of the transducerelements at a variable phase while driving the other transducer elementsat a constant phase, thereby varying the focus quality; (ii) determininga phase difference, if any, between the constant and variable phaseswhere the focus quality is optimized; and (iii) if the phase differenceis non-zero, adjusting a relative phase of the selected transducerelement based thereon.
 14. The method of claim 13 further comprisingrepeating steps (i) through (iii) for the remaining transducer elements.15. The method of claim 1 further comprising the step of providing aphantom, the driving step generating an ultrasound focus in the phantomand the measuring step measuring a focus quality in the phantom.
 16. Themethod of claim 15 wherein the phantom comprises a material having lowtensile strength.
 17. The method of claim 16 wherein the phantomcomprises a material having a small elastic modulus.
 18. A method formapping an ultrasound transducer comprising a plurality of transducerelements, the method comprising: (a) driving the plurality of transducerelements so as to generate an ultrasound focus; (b) using acousticradiation force imaging, measuring a displacement associated with thefocus; (c) driving a selected one of the transducer elements at avariable phase while driving the other transducer elements at a constantphase, thereby varying the displacement; (d) determining a phasedifference, if any, between the constant and variable phases where thedisplacement is maximized; and (e) if the phase difference is non-zero,adjusting a relative phase of the selected transducer element basedthereon.
 19. A method for controlling an ultrasound transducercomprising a plurality of transducer elements, the method comprising:(a) mapping the ultrasound transducer by (i) driving the plurality oftransducer elements so as to generate an ultrasound focus, (ii) varyinga focus-affecting parameter associated with at least one of the elementsand measuring a resulting variation on a quality of the focus, and (iii)selecting a value of the parameter associated with a best focus quality;and (b) controlling the transducer based on the mapping step.
 20. Themethod of claim 19 wherein controlling the transducer comprises drivingthe elements to produce outputs converging at a focus corresponding to atarget treatment region.
 21. The method of claim 19 wherein the mappingstep comprises generating the ultrasound focus in a phantom andmeasuring the focus quality in the phantom.
 22. The method of claim 19further comprising, following step (b), repeating the mapping step. 23.The method of claim 19 wherein substep (ii) comprises, for each of thetransducer elements, determining a relative phase associated with thetransducer element by (1) driving the transducer element at a variablephase while driving the other transducer elements at a constant phase,thereby varying the focus quality, and (2) determining a phasedifference, if any, between the constant and variable phases where thefocus quality is optimized.
 24. The method of claim 19, wherein thefocus quality is measured using acoustic radiation force imaging tomeasure a displacement associated with the focus.
 25. A method formapping an ultrasound transducer comprising a plurality of transducerelements, the method comprising: (a) driving the plurality of transducerelements so as to generate an ultrasound focus; (b) using acousticradiation force imaging, measuring a displacement associated with thefocus; (c) grouping the transducer elements; (d) driving a selectedgroup of transducer elements at a variable phase while driving the othergroups of transducer elements at a constant phase, thereby varying thedisplacement; (e) determining a phase difference, if any, between theconstant and variable phases where the displacement is maximized; and(f) if the phase difference is non-zero, adjusting a relative phase ofthe selected group of transducer elements based thereon.